MCAT Biology Review
Chapter 11: The Musculoskeletal System
One concept that has been emphasized throughout the past eight chapters focusing on anatomy and physiology is the notion that organ systems work together in order to achieve a desired effect. The musculoskeletal system is no different. Usually, we think of the musculoskeletal system as being responsible for movement, but to limit the musculoskeletal system to that function would be shortsighted. The bones are reservoirs of calcium and other minerals that can be released through hormonal signaling. They protect the internal organs and provide support for the body. Muscle tissue not only moves these bones, but pumps blood through the body and regulates the function of a number of other systems, including respiration, digestion, blood pressure and vascular tone, and reproduction and childbirth. As you continue reviewing anatomy and physiology and master the fundamentals of each organ system, be sure to pay special attention to how each organ system interacts with the others. While the MCAT expects you to understand each individual organ system, it will also challenge your knowledge by asking you to think critically about how one system impacts another. By spending some time in your studies looking at these interactions, you will be one step ahead on Test Day.
In our final chapter of MCAT Biology Review, we will switch gears and look at the transfer of information from generation to generation. This picks up on a discussion from the first three chapters of the book in which we explored the organization of cells and their genetic material, reproduction, and embryogenesis and development. In the next chapter, we’ll describe classical (Mendelian) inheritance and conclude with a note on how the gene pool can change over time with the topic of evolution.
The Muscular System
· There are three main types of muscle: skeletal muscle, smooth muscle, and cardiac muscle.
o Skeletal muscle is involved in support and movement, propulsion of blood in the venous system, and thermoregulation. It appears striated, is under voluntary (somatic) control, is polynucleated, and can be divided into red (slow-twitch) fibers that carry out oxidative phosphorylation and white (fast-twitch) fibers that rely on anaerobic metabolism.
o Smooth muscle is in the respiratory, reproductive, cardiovascular, and digestive systems. It appears nonstriated, is under involuntary (autonomic) control, and is uninucleated. It can display myogenic activity, or contraction without neural input.
o Cardiac muscle comprises the contractile tissue of the heart. It appears striated, is under involuntary (autonomic) control, and is uninucleated (sometimes binucleated). It can also display myogenic activity. Cells are connected with intercalated discs that contain gap junctions.
· The sarcomere is the basic contractile unit of striated muscle.
o Sarcomeres are made of thick (myosin) and thin (actin) filaments.
o Troponin and tropomyosin are found on the thin filament and regulate actin–myosin interactions.
· The sarcomere can be divided into different lines, zones, and bands.
o The boundaries of each sarcomere are defined by Z-lines.
o The M-line is located in the center of the sarcomere.
o The I-band contains only thin filaments.
o The H-zone consists of only thick filaments.
o The A-band contains the thick filaments in their entirety. It is the only part of the sarcomere that maintains a constant size during contraction.
· Sarcomeres attach end-to-end to become myofibrils, and each myocyte (muscle cell or muscle fiber) contains many myofibrils.
o Myofibrils are surrounded by the sarcoplasmic reticulum, a calcium-containing modified endoplasmic reticulum, and the cell membrane of myocytes is known as the sarcolemma.
o A system of T-tubules is connected to the sarcolemma and oriented perpendicularly to the myofibrils, allowing the incoming signal to reach all parts of the muscle.
· Muscle contraction begins at the neuromuscular junction, where the motor neuron releases acetylcholine that binds to receptors on the sarcolemma, causing depolarization.
o This depolarization spreads down the sarcolemma to the T-tubules, triggering the release of calcium ions.
o Calcium binds to troponin, causing a shift in tropomyosin and exposure of the myosin-binding sites on the actin thin filament.
o Shortening of the sarcomere occurs as myosin heads bind to the exposed sites on actin, forming cross bridges and pulling the actin filament along the thick filament, resulting in contraction. This is known as the sliding filament model.
o The muscle relaxes when acetylcholine is degraded by acetylcholinesterase, terminating the signal and allowing calcium to be brought back into the SR. ATP binds to the myosin head, allowing it to release from actin.
· Muscle cells exhibit an all-or-nothing response called a simple twitch.
o Addition of multiple simple twitches before the muscle has an opportunity to fully relax is called frequency summation.
o Simple twitches that occur so frequently as to not let the muscle relax at all can lead to tetanus, a more prolonged and stronger contraction.
· Muscle cells have additional energy reserves to reduce oxygen debt (the difference between the amount of oxygen needed and the amount present) and forestall fatigue.
o Creatine phosphate can transfer a phosphate group to ADP, forming ATP.
o Myoglobin is a heme-containing protein that is a muscular oxygen reserve.
The Skeletal System
· Internal skeletons (like those in humans) are called endoskeletons; external skeletons (like those in arthropods) are called exoskeletons.
· The human skeletal system can be divided into axial and appendicular skeletons.
o The axial skeleton consists of structures in the midline such as the skull, vertebral column, ribcage, and hyoid bone.
o The appendicular skeleton consists of the bones of the limbs, the pectoral girdle, and the pelvis.
· Bone is derived from embryonic mesoderm and includes both compact and spongy (cancellous) types.
o Compact bone provides strength and is dense.
o Spongy or cancellous bone has a lattice-like structure consisting of bony spicules known as trabeculae. The cavities are filled with bone marrow.
o Long bones contain shafts called diaphyses that flare to form metaphyses and that terminate in epiphyses. The epiphysis contains an epiphyseal (growth) plate that causes linear growth of the bone.
o Bone is surrounded by a layer of connective tissue called periosteum.
o Bones are attached to muscles by tendons and to each other by ligaments.
· Bone matrix has both organic components, like collagen, glycoproteins, and other peptides, and inorganic components, like hydroxyapatite.
o Bone is organized into concentric rings called lamellae around a central Haversian or Volkmann’s canal. This structural unit is called an osteon or Haversian system.
o Between lamellar rings are lacunae, where osteocytes reside, which are connected with canaliculi to allow for nutrient and waste transfer.
· Bone remodeling is carried out by osteoblasts and osteoclasts. Osteoblasts build bone, while osteoclasts resorb bone.
o Parathyroid hormone increases resorption of bone, increasing calcium and phosphate concentrations in the blood.
o Vitamin D also increases resorption of bone, leading to increased turnover and, subsequently, the production of stronger bone.
o Calcitonin increases bone formation, decreasing calcium concentrations in the blood.
· Cartilage is a firm, elastic material secreted by chondrocytes. Its matrix is called chondrin.
o Cartilage is usually found in areas that require more flexibility or cushioning.
o Cartilage is avascular and is not innervated.
· In fetal life, bone forms from cartilage through endochondral ossification. Some bones, especially those of the skull, form directly from undifferentiated tissue (mesenchyme) in intramembranous ossification.
· Joints may be classified as immovable or movable.
o Immovable joints are fused together to form sutures or similar fibrous joints.
o Movable joints are usually strengthened by ligaments and contain a synovial capsule.
o Synovial fluid, secreted by the synovium, aids in motion by lubricating the joint.
o Each bone in the joint is coated with articular cartilage to aid in movement and provide cushioning.
· Muscles that serve opposite functions come in antagonistic pairs; when one muscle contracts, the other lengthens.
Answers to Concept Checks
1. Skeletal and cardiac muscle are striated. Smooth muscle is always uninucleated. Skeletal muscle is always polynucleated. Skeletal muscle is voluntary. Smooth and cardiac muscle are innervated by the autonomic nervous system. Smooth and cardiac muscle exhibit myogenic activity.
2. The A-band does not change length during muscle contraction because it is the entire length of the myosin filament. The filaments do not change length, but rather slide over each other; thus, the A-band should remain a constant length during contraction.
3. Release of acetylcholine from motor neuron → activation of acetylcholine receptors in sarcolemma → depolarization of sarcolemma → spreading of signal using T-tubules → release of calcium from sarcoplasmic reticulum (SR) → binding of calcium to troponin → conformational shift of tropomyosin → exposure of myosin-binding sites → myosin binds to actin
4. ATP binding allows the myosin filament to disconnect from actin. Dissociation of ADP and inorganic phosphate from myosin causes the powerstroke.
5. Tetanus is the summation of multiple simple twitches that occur too quickly for the muscle to relax. This leads to a stronger and more prolonged contraction of the muscle.
1. Compact bone is dense and is used for its strength; it forms most of the outer layers of a bone. Spongy (cancellous) bone has many spaces between bony spicules called trabeculae and is the site of marrow production. It is found in the interior core of the bone and also helps distribute forces or pressures on the bone.
2. The three parts of a bone are the diaphysis, metaphyses, and epiphyses. Growth plates are found in epiphyses and contribute to linear growth.
3. Most inorganic bone is composed of hydroxyapatite crystals.
4. Osteoblasts build bone. Osteoclasts chew bone (break it down). Chondrocytes form cartilage.
5. Synovial fluid, produced by the synovium, lubricates movable joints.
· Biology Chapter 4
o The Nervous System
· Biology Chapter 5
o The Endocrine System
· Biology Chapter 6
o The Respiratory System
· Biology Chapter 7
o The Cardiovascular System
· General Chemistry Chapter 9
· Physics and Math Chapter 1
o Kinematics and Dynamics